Changes in marine iguana (Amblyrhynchus cristatus) heart rates suggest reduced metabolism during El Niño events

Marine iguanas occasionally face severe food shortages because of algal dieback during El Ni & ntilde;o events. Research on their adaptations to these periods has highlighted their unique ability to shrink in body length, which reduces their energetic needs. Additional mechanisms, like sustaining lower body temperatures and metabolic rates, could potentially also lower energy consumption, but have never been examined. We measured 665 iguanas over an 11-year period including three El Ni & ntilde;o events, and examined how heart rates (a proxy for metabolic rates) and body temperatures change with sea-surface temperature oscillations (Oceanic Ni & ntilde;o Index, ONI). Heart rate (adjusting for body size, temperature, season, and study site) was negatively correlated with ONI and lower during El Ni & ntilde;o, whereas the adjusted body temperature did not correlate with ONI or differ between El Ni & ntilde;o and other periods. We therefore hypothesize that marine iguanas can depress their metabolic rates in response to the harsh conditions, an adaptation that is complementary to shrinking and may further enhance their survival through periods of limited food. Direct metabolic measurements are needed to test this hypothesis.